Applicant claims priority under 35 U.S.C. xc2xa7119 of German Application No. 101 52 865.5 filed Oct. 25, 2001.
1. Field of the Invention
The present invention relates to a multi-layer film produced by co-extrusion, which is particularly intended for use as a labeling film, a laminating film or a packaging film.
2. The Prior Art
A multi-layer packaging film is known from U.S. Pat. No. 6,060,136. This film includes two polyolefinic outer layers and a core consisting of a mixture of styrene-homopolymers and styrene-butadiene-styrene block polymers (SBS-block copolymer). The core may have a multi-layer structure in which at least one layer consists of a mixture of styrene homopolymers and SBS block copolymers. The content of styrene homopolymers in the mixture amounts to 50% by weight at the most, and the preferred range is in the range of 10% and 20% by weight.
It is an object of the invention to provide a film with enhanced rigidity that is suitable as a labeling film or a laminating film and can be easily processed. When used for packaging, the film should exhibit improved dead-fold and twisting properties.
The above objects are accomplished with a multi-layer film produced by co-extrusion that includes two outer layers made of a polyolefin, a polyolefin mixture or an olefin copolymer, and a core that consists of a mixture of styrene homopolymers and styrene-butadiene-styrene block copolymers at least in the areas bordering on the outer layers. The mixture contains more than 50% by weight up to a maximum of 90% by weight styrene homopolymers, the balance of the mixture being styrene-butadiene-styrene block copolymers. The film preferably has a thickness of between 20 and 100 xcexcm and can be used as a labeling, laminating or packaging film. Because the film in accordance with the invention has a high styrene content near the polyolefinic outer layers, the film exhibits good stiffness properties. The addition of styrene-butadiene-styrene block copolymers (SBS block copolymers) is required in order to produce the adhesion to the polyolefinic outer layers. Surprisingly low amounts of SBS block copolymers are adequate for this purpose. To that extent, the content of SBS block copolymers in the areas bordering on the outer layers amounts only to between 10% by weight and less than 50% by weight.
The core may be made up of only one single layer with a homogeneous composition of the mixture. The thickness ratio of the layers can be adapted to the purpose of application. The ratio of thickness between one of the outer layers and the core can be selected between 1:1 and 1:6. For many applications, the thickness of the core amounts to about three times the thickness of an outer layer.
According to a preferred embodiment of the invention, the core has three layers, so that a five-layer structure is obtained overall. Various possibilities are available for forming the layers. According to one embodiment, the inner layer of the core is made up of a polyolefin, a polyolefin mixture or an olefin copolymer, and the layers bordering on the core on the two sides of the inner layer are made up of a mixture of styrene-homopolymers and styrene-butadiene-styrene block copolymers with more than 50% by weight styrene homopolymer and at least 10% by weight styrene-butadiene-styrene block copolymer. This embodiment is especially advantageous for films with a high film thickness. With a low amount of styrene homopolymer material used based on the weight of the film, a multi-layer film is formed that has good rigidity, high transparency and a good tear propagation property in the transverse direction. In addition, the film remains flexible and deformable for further processing. With a film having this layer structure, surprisingly good rigidity is obtained due to the high concentrations of the styrene homopolymers near the outer layers.
In another embodiment that is primarily intended for thinner foils, the core is made up of three layers each made up of a mixture of styrene homopolymers and styrene-butadiene-styrene block copolymers. The inner layer has a higher concentration of styrene homopolymers than the layers bordering on it on both sides. Due to the step-wise increase in concentration of styrene homopolymers from the outside inwards, a film may be obtained with both high rigidity and good adhesion of the polyolefinic outer layers. The inner layer of the core may be formed also by pure styrene homopolymers. With this layer structure, thin films with high rigidity can be produced.
For embodiments having a 5-layer structure, the layers in the film may be have a thickness ratio between 1:1:1:1:1 and 6:1:6:1:6.
The outer layers preferably are made up of polyethylene or an ethylene copolymer. For example, the outer layers may be formed from linear low density polyethylene (LLDPE) which is particularly easy to process and is also well-suited as a sealing layer.
At least one of the layers may additionally contain pigments and/or fillers. Furthermore, an additional layer may be applied to one of the outer layers. Suitable for this purpose are, for example, adhesive layers or layers suitable for imprinting. Moreover, the outer layers may be after-treated physically (e.g. by a corona treatment), or also chemically, in the manner known per se, so that their adhesive and/or their imprinting properties are enhanced.
The multi-layered film as defined by the invention has a broad spectrum of application. It is suitable as a labeling film and a packaging film, and in this area it is especially useful as a twisting film for candies. The film may also be used to form a packaging pouch, and also may be used as a laminating film. As opposed to blended films using mixtures of polyolefins and polystyrene, the film in accordance with the invention possesses high transparency, which is an advantage for many applications.